In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is synchronization.
For example, when a wireless device, such as a user equipment (UE) is powered on, or when it moves between cells in a cellular wireless radio access network, it receives and synchronizes to downlink signals (as transmitted by radio access network nodes of the cellular wireless radio access network) in a cell search procedure. One purpose of this cell search procedure is to identify the best cell for the wireless device and to achieve time and frequency synchronization to the network in the downlink (i.e. from radio access network node to wireless device).
So-called Primary and Secondary Synchronization Signals (PSS and SSS) as defined in Section 6.11 of 3GPP TS 36.211, version 12.5.0, can, for Long Term Evolution (LTE), be used at cell search as performed by the wireless device, see section 4.1 in 3GPP TS 36.213, V 12.5.0. Here, in the case of frequency-division duplexing (FDD), the PSS is transmitted in the last orthogonal frequency-division multiplexing (OFDM) symbol of slots 0 and 10 within a frame and the SSS is transmitted in the OFDM symbol preceding the PSS. In the case of time-division duplexing (TDD), the PSS is transmitted in the third OFDM symbol of slots 3 and 13 within a frame, and the SSS is transmitted in the last OFDM symbol of slots 2 and 12, i.e., three symbols ahead of the PSS.
A simplified initial cell search procedure between a second network device 300 of a wireless device 130 and a first network device 200 of a radio access network node 120 is illustrated in the signalling diagram of FIG. 9.
S901: The wireless device 130 is powered “on”.
S902: The radio access network node 120 transmits PSS.
S903: The wireless device 130 receives the PSS and determines therefrom a physical layer ID within a group (1 out of 3 groups), OFDM symbol synchronization, and coarse frequency offset estimation. Here the wireless device tries to detect PSS from which the wireless device can derive the cell identity (ID) within a cell-identity group, which consists of three different cell identities corresponding to three different PSS. In this detection, the wireless device thus has to blindly search for all of these three possible cell identities. The wireless device also achieves OFDM symbol synchronization and a coarse frequency offset estimation with an accuracy of about 1 kHz. The latter is estimated by the wireless device by evaluating several hypotheses of the frequency error.
S904: The radio access network node 120 transmits SSS.
S905: The wireless device 130 receives the SSS and determines therefrom a physical cell ID, radio frame synchronization, cyclic prefix length, duplex mode (and, optionally, fine frequency offset estimation). The wireless device uses coherent detection of the SSS thanks to the PSS decoding from which the wireless device acquires the physical cell ID and achieves radio frame synchronization. Here, the wireless device also detects if normal or extended cyclic prefix is used. If the wireless device is not preconfigured for TDD or FDD, the wireless device can detect the duplex mode (TDD or FDD) by the position in the frame of detected SSS in relation to detected PSS. Fine frequency offset estimation can be estimated by correlating PSS and SSS.
S906: The radio access network node 120 transmits cell specific reference signals (CRS).
S907: The wireless device 130 receives the CRS and determines therefrom channel estimation (and, optionally, fine frequency offset estimation). The fine frequency offset estimation can thus alternatively be estimated by the wireless device using the Cell specific Reference Signals (CRS) which are derived from the Physical Cell Identity (PCI) encoded in the PSS/SSS.
S908: The radio access network node 120 transmits a physical broadcast channel (PBCH).
S909: The wireless device 130 receives the PBCH and decodes therefrom a master information block (MIB). Once the wireless device is capable of processing the CRSs the wireless device can thus receive and decode cell system information which contains cell configuration parameters starting with the PBCH.
The PSS and the SSS are examples of reference signals. Two examples of how such reference signals can be transmitted will be disclosed below with reference to FIGS. 10 and 11.
The radio access network within LTE is based on transmission of OFDM symbols in downlink and Discrete Fourier Transform (DFT) spread OFDM (also known as single carrier frequency division multiple access, SC-FDMA) in the uplink. In DFT-spread OFDM the signal to be transmitted in the uplink is pre-coded by a DFT, mapped to a frequency interval in which it is allocated, transformed to the time domain, concatenated with a cyclic prefix and then transmitted over the radio interface. The DFT spread OFDM scheme as used in uplink has significantly lower Peak to Average Power Ratio (PAPR) as compared to OFDM. By having a low PAPR, the transmitter can be equipped with simpler and less energy consuming radio equipment, which is beneficial for wireless devices where cost and battery consumptions are can be issues.
FIG. 10 is a schematic illustration of reference signal allocation of two related synchronization signals 1A and 1B in a time frequency grid according to a first example. Two OFDM symbols are shown along the time axis and N1+1, where N1 is an integer, number of sub-carriers are shown along the frequency axis. Synchronization signal 1A is a first synchronization signal and synchronization signal 1B is a second synchronization signal. This first synchronization signal 1A can, for example, be a PSS and the second synchronization signal 2B could be a SSS. If beamforming is used, both synchronization signals 1A and 1B are transmitted with one configuration of the beamforming. Similar to the FDD mode in LTE release 8, the synchronization signals 1A and 1B are transmitted in adjacent OFDM symbols number 0 and 1.
Another beamforming configuration can be used to transmit the synchronization signals in OFDM symbols 2, 4, . . . . As described earlier, the LTE release 8 synchronization procedures starts with OFDM symbol synchronization by PSS detection. In order to keep low computational complexity, the same synchronization signal is used for both OFDM symbol 0 and 2 but with different beamforming configurations. The wireless device can then detect either the synchronization signal in OFDM symbol 0 or 2, from the best detection of the sequence used in both OFDM symbol 0 or 2.
The wireless device can then continue to detect the second synchronization signal 1B in OFDM symbols 1, 3, . . . , which can be different sequences. Here, the wireless device can detect the second synchronization signal 1B in OFDM symbol 2 or 3 after having detected the first synchronization signal 1A. After detecting the second synchronization signal 1B, the wireless device knows the symbol index, if different second synchronization signals 1B are used in different OFDM symbols. The symbol index is thus implicitly signaled by the second synchronization signal 1B.
This approach can be extended to any number of first synchronization signals, as transmitted in OFDM symbols 0, 2, 4, . . . followed by second synchronization signals, as transmitted in OFDM symbols 1, 3, 5, . . . . By placing the first synchronization signal 1A and the second synchronization signal 1B in different OFDM symbols, the same beamforming is always used in two consecutive OFDM symbols.
FIG. 11 is a schematic illustration of reference signal allocation of four related synchronization signals 1A, 1B 2A, 2B in a time frequency grid according to a second example. Two OFDM symbols are shown along the time axis and N2+1, where N2>N1 is an integer, number of sub-carriers are shown along the frequency axis.
According to the reference signal allocation of FIG. 11 a mapping of the first synchronization signal 1A and the second synchronization signal 1B is made to the same OFDM symbol, but using different sub-carriers. Here, half of the number of OFDM symbols can be used as compared to the approach in FIG. 10, but instead twice the number of sub-carriers needs to be allocated. Since the beamforming is performed per OFDM symbol basis, the time required to transmit in a specific number of beamforming candidates is half as compared to the approach in FIG. 10.
At least two OFDM symbols are needed for the first and second synchronization signals for the approach in FIG. 10. In future communications systems with beamforming, the synchronization signals will have to be beamformed with several beamforming candidates which increases the number of symbols needed for synchronizations. For analog beamforming, each OFDM symbol used for synchronization signal with a specific beamforming requires hardware support. Assuming beams transmitted in several directions, longer delays proportional to the number of directions can be reached; i.e., M beam directions, where M>1 is an integer, basically means a delays up to two times M per antenna array for the approach in FIG. 10.
If the reference signal allocation of FIG. 11 is used, the first synchronization signal 1A and the second synchronization signal 1B are mapped to the same OFDM symbol, but different sub-carriers (and likewise for the first synchronization signal 2A and the second synchronization signal 2B). Despite the potential to reduce the delay to acquire synchronization since the first and second synchronization signals are mapped to different sub-carriers, the single carrier property of the transmission is violated, thus resulting in higher PAPR in the transmitter.
Hence, there is still a need for an improved transmission and reception of m reference signals.